Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include transmitters having a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Receivers are also provided to demultiplex the received WDM signal into individual optical signals, convert the optical signals into electrical signals, and output the data carried by those electrical signals.
Conventionally, WDM systems have been constructed from discrete components. For example, demultiplexer and photodiodes have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components, have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated optical output.
One such optical signal modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide spectral densities with higher data rates per unit of fiber bandwidth than other modulation formats, such as on-off keying (OOK).
A receiver configured to decode and output the information carried by a Pol Muxed DQPSK signal is described in U.S. patent application Ser. Nos. 12/052,541; 12/345,817; and 12/345,824, the entire contents of each of which are incorporated herein by reference. In such systems, various components are provided on multiple substrates. There is a need, however, to provide such components of an optical receiver, which receives Pol Muxed DQPSK modulated optical signals or optical signals modulated in accordance with other modulation formats, on one substrate to improve reliability, simplify manufacturing, and reduce costs.
In addition, variable optical attenuators (VOAs) may be provided to adjust the power levels of optical signals received by the receiver or routed within the receiver. Such VOAs may also output a smaller amount of residual or extraneous light that may interfere with the power adjusted signals. Thus, there is also a need to provide VOAs that output optical signals with less interference or cross-talk.